The invention relates to a spray unit, downstream of and in-line with an extrusion press, for cooling metallic extruded sections from all sides, in particular sections of aluminum or an aluminum alloy, and featuring a cooling medium that is program-controlled, directed at the extrusion from spray nozzles and such that the distance, the cross-sectional distribution and the direction of the spray nozzles are adaptable specific to the geometrical shape of the section and the distribution of mass in the section, and the spray nozzles are arranged in nozzle beams running in the direction of the extruded section. The invention relates further to a process for optimal setting of the spray nozzles and to a process for operating a spray unit.
When cooling extruded sections, wherein rapid cooling is also referred to as quenching, large sections of irregular shape and mass distribution present problems in that the individual cross-sectional regions have different thermal capacities. Rapid cooling of thin cross-sections and slow cooling of thick cross-sections produce stresses that can lead to considerable deformation and consequently to repair work or even scrap.
Viewed from that standpoint, it would appear that slow cooling is desirable. At the same time, however, slow cooling leads to coarse grains. The section becomes too weak and has poor mechanical properties. If, on the other hand, the section is quenched, then the existing structure is frozen in, and the small grains obtained offer advantages with respect to mechanical properties. The cooling rate is therefore an important parameter in extrusion technology and must be optimized in such a way that on the one hand no permanent deformation occurs, or if so then only to a degree permitted by the given tolerance limits, and on the other hand such that grain growth is limited to such an extent that adequate mechanical properties are retained.
Cooling a section as it emerges from the extrusion press depends not only on the amount, thermal capacity and delivery of the cooling medium, but also on the rate at which the extrusion emerges from the press. If this extrusion speed is very low, then a cooling medium of low thermal capacity such as air can be employed. In general somewhat too slow cooling is less of a disadvantage than too fast cooling as slightly reduced mechanical strength is more tolerable than a distorted section. The use of air as a cooling medium however has its limits, as the productivity of the extrusion process is reduced correspondingly with the extrusion speed and air cooling can not be employed for all alloy compositions. Air is therefore not an optimal cooling medium.
Quenching in a water bath or by spraying water onto the extrusion on the other hand often results in too strong cooling. An irregular section distorted by non-uniform cooling over its cross-section has to be straightened again by use of very large forces, which is complicated and expensive. The process often has to be repeated several times over.
Compared to an air stream with a cooling capacity of about 100 to 150 W/m.sup.2..degree.C., a water bath with forced circulation offers a cooling capacity of 20 to 50 kW/m.sup.2..degree.C., and a water jet as much as 20-100 kW/m.sup.2..degree.C. In a water bath, however, in spite of strong stirring, local bubbles of steam form; this strongly reduces the cooling action locally and plastic deformation can occur. Using a water jet this problem can be diminished but not completely prevented.
By spraying an air-water mixture it is possible to practically eliminate the above mentioned problem completely. Using an optimum amount of water makes it possible for the fine droplets to vaporize completely. That way a heat transfer or cooling action of 0.5 to 30 kW/m.sup.2..degree.C. is achieved. The sprayed air-water mixture can easily penetrate the remotest corner or smallest undercut on the surface of the section without the risk of steam bubbles forming. A spray of air-water mixture can be used satisfactorily for cooling practically all alloys at moderate to slow cooling rates.
Described in EP-A-0 429 394 is a process and a device for cooling cast ingots of aluminum after homogenization. The slabs emerging at a first temperature are led from a soaking furnace, continuously in the longitudinal direction one after another "in-line", at a program-controlled rate of advance through a spray unit where they undergo program controlled spray cooling by a cooling medium from all sides to reach a preselected surface temperature. The surface and the interior of the slabs reach the same temperature shortly after leaving the spray unit. The spray unit is conceived such that it is fitted with groups or individual adjustable nozzles for the cooling medium over its whole length and over the whole periphery, of its interior. These nozzles are preferably designed according to EP-A-0 343 103 and permit a pendular movement of the spray cone, which effects more uniform cooling over the total surface area.
The slabs cooled in accordance with EP-A-0 429 394 are always regular in cross-section, for example, rectangular, square or round. The arrangement of the nozzles in the spray unit is conceived accordingly. The knowledge and experience from EP-A-0 429 394 relate to regularly shaped slabs, e.g. in the shape of rectangular, square or round slabs. In the case of sections that are irregular in cross-section and mass distribution there arise the above mentioned problems that cause the sections to emerge distorted from the spray unit.
A spray unit with tiltable spray nozzles arranged on nozzle beams is in principle known from DE-U-88 10 085.